The only thing you can do to prevent your son from emulating his father - is to present to him another role model of a NON-narcissist - YOU. Hopefully, when he grows up, he will prefer your model to his father's. But there is only that much you can do. You cannot control the developmental path of your son. Exerting unlimited control over your son is what narcissism is all about - and is exactly what you should avoid at all costs, however worried you might be.
Narcissism does tend to breed Narcissism - but not inevitably. Not all the off-spring of a narcissist inexorably become narcissists.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase: http://www.narcissistic-abuse.com/thebook.html)